1. Field of Invention
This invention relates to optical processors and more particularly to actuatable, diffractive optical processors.
2. Related Art
Microelectromechanical manufacturing techniques facilitate batch fabrication of microelectromechanical systems (or “MEMS”) that have complex features and functions. Microscale sensing and actuation applications are particularly well addressed by MEMS.
For many MEMS applications, electrostatically-actuated structures are particularly effective as analog positioning and tuning components. Electrostatic actuation provides a combination of advantages for the microscale size regime of MEMS, including the ability to produce high energy densities and large force generation, as well as high operational speed, and the general ease of fabrication.
Electrostatic actuation of a structure is typically accomplished by applying a voltage between an electrode on the structure and an electrode separated from the structure. The resulting attractive electrostatic force between the electrodes enables actuation of the structure toward the separated electrode. This applied elecrostatic force is opposed by a mechanical restoring force that is a function of the structure's geometrical and material properties. Controlling the structure's position during actuation requires balancing the applied electrostatic force and mechanical restoring forces. A more detailed description of the forces existing in an electrostatic device and design applications thereof is given in numerous publications and, in part, in U.S. patent application Ser. No. 09/537,936 entitled “PRECISION ELECTROSTATIC ACTUATION AND POSITIONING,” filed on Mar. 29, 2000 in the name of Elmer S. Hung, et al., which is hereby incorporated by reference.
MEMS have been used in numerous ways as optical processors, including uses as diffractive optical processors. Examples of applications of diffractive optical processors have included optical communications applications, and metrologic applications such as polychomators used in spectroscopic systems. In such applications, the actuatable structures of the MEMS optical processor device is constructed to function as an actuatable grating structure. Because the performance of grating-based optical processors is effected by movements of the grating structure on the order of hundredths of the wavelength of light to be processed by the device, the precise positioning of the structure is critical. If precise positioning is not maintained over the entire surface of the optical processor, the useable portion of the surface is limited to the portions that are precisely positioned.
An example of a diffractive optical processor is disclosed in U.S. Pat. No. 5,311,360 titled “Method and Apparatus for Modulating a Light Beam” issued May 10, 1994, by Bloom, et al. The optical processor disclosed by Bloom, et al. has a plurality of grating elements; each grating element is connected to a frame at both ends, but otherwise forming a free standing bridge between the two frame connections. An electrode is placed below each of the grating elements such that when a voltage is provided between a grating element and a corresponding electrode, the grating element is deflected toward the electrode. Because the grating element is connected at the ends, the deflected grating element forms a continuous curve, with the maximum deflection of the electrode occurring at the midpoint between the connections, and zero deflection occurring at the connections.
While the processor is able to achieve a selected deflection near the midpoint of the grating elements, the deflection of the remaining portions of the grating element is determined by the properties of the material from which the grating element made, and the distance between the connections. Accordingly, the useable portions of each of the grating elements in the optical processor is limited to a portion near the midpoints of the grating elements, where the surface has the selected deflection, and the grating approaches an appropriate flatness and orientation.
Other MEMS grating structures have allowed the grating elements to remain nearly planar during actuation, but structures necessary to maintain planarity have resulted in limitations in optical performance. FIG. 1 is a schematic top view of a MEMS diffractive optical processor 100 that illustrates a basic topography for prior art MEMS diffractive optical processors that maintains planarity of the grating elements during actuation. The top surface 101 of optical processor 100 includes gaps 115 and actuatable grating elements 110. Grating elements 110 are actuatable in the direction of the z-axis (i.e., perpendicular to the 30 top surface of optical processor 100) to control the diffractive characteristics of optical processor 100. Gaps 115 are fixed regions of the top surface of optical processor 100 that provide separation between grating elements 110, thus allowing actuation of neighboring grating elements 110 without mechanical interference.
The performance characteristics of diffractive optical processors, such as prior art diffractive optical processor 100, are affected by diffraction and scattering by the gaps 115. Accordingly, prior art MEMS-based optical processors having such a topography have had limitations in insertion loss, and dynamic range.
The following terms and phrases will have the following definitions throughout this specification. “Insertion Loss” is a measure of device efficiency, defined as the loss of optical energy in an optical signal, resulting from transmission by an optical device. Insertion loss is a measure of the total signal energy output from a device relative to the total signal energy input into the device, often expressed in decibels.
“Dynamic Range” of an analog device is a measure of the range of signal strengths over which a device can operate. Dynamic range is the span between the maximum signal strength attainable at the device output and the minimum signal strength attainable at the device output.
In many applications, it is desirable to maintain the strength of an input signal, independent of the polarization of the signal. The degree to which an optical device attenuates an input signal as a function of polarization is referred to as “Polarization-Dependent Loss” (“PDL”). Ideally, the PDL is zero.